RESPIRATION IN AMPHIBIA. 869 



frequently even two sets of these organs. The first set are called the eorternal 

 gills, and consist of soft processes slightly branched, or very much subdivided, 

 or even plumose ; they are attached to the side of the neck and project freely 

 into the water. In the larva of the higher or tailless Amphibia, the frogs and 

 toads, the external gills remain only for a few days ; but in the tailed sala- 

 manders or newts, they exist for a longer period, and in the lowest Amphibia 

 they persist throughout life. They are, at least when newly developed, al- 

 ways covered with cilia, and each minute subdivision or branch contains a 

 looped capillary vessel, one side of which conveys outwards the deoxygenated 

 venous blood, and the other inwards the oxygenated or arterial blood. The 

 second set of gills, found in certain Amphibia only, are named the internal 

 gills; these appear in frogs and toads after the wasting of the external gills ; 

 they consist of minute fringes of vascular processes attached to the cartilagin- 

 ous branchial arches of the hyoid apparatus, and are protected by a fold of 

 the skin of the neck, so as to lie in a sort of branchial chamber, which commu- 

 nicates with the pharynx ; the opening on the right side of the neck very 

 early becomes closed. Water reaches these internal gills, by flowing in through 

 the mouth or nose, and then out through a small orifice on the left side of the 

 neck, the movement of the water being caused by an act resembling that of 

 deglutition. It is these internal gills, found in some of the Amphibia, which 

 are the representatives or homologues of the gills of Fishes ; they are not de- 

 veloped in Menobranchus and Amphiuma. In the frogs, toads, and true 

 salamanders, both sets of gills disappear ; hence these are named Caducibranch- 

 iate Amphibia ; they afterwards breathe by the lungs and skin only. In the 

 salamandroid Perennibranchiate Amphibia, including the Axolotl, Meno- 

 branchus, Siren, and Proteus, the gills are persistent ; they are really the ex- 

 ternal gills, though they may become attached, as in the Axolotl, to the first 

 branchial arch of the hyoid apparatus. 



In Fishes, the branchiae or gills, which are always internal or covered, at- 

 tain their highest development and most complex forms. They usually con- 

 sist of numerous comb-like processes, supported, like single or double fringes, 

 on the branchial arches of the hyoid apparatus, and forming four or five lam- 

 inae on each side of the pharynx. They are, in some cases, as in the Carti- 

 laginous Fishes, concealed by the integuments ; but in others, as in the Os- 

 seous Fishes, by a movable osseous and cutaneous covering, named the oper- 

 culum. In the latter case, only one external gill opening exists on each side, 

 the operculum overlapping all the branchial arches ; but internally the gill- 

 chamber opens into the pharynx, by separate clefts or apertures between the 

 branchial arches. In the former case, however, the gill-chamber is completely 

 divided into passages, varying from three to seven in number, each having a 

 separate internal and external aperture, corresponding with the clefts or spaces 

 between the branchial arches. In some of the lowest forms, the gills are mere 

 folds of membrane, lining distinct sacs. In the Myxine, the branchial outlets 

 unite into a single canal, which runs backwards, and opens on the under sur- 

 face of the fish, at a sort of abdominal pore. 



The extent of surface obtained by the comb-like fringes of the gills of Fishes 

 for exposure to water, is very large, especially in the rays and skates. The 

 water drawn into the opened mouth is forced, at intervals, by the hyoid and 

 pharyngeal muscles, between and over the gills, in the direction of their 

 fringes, and is rapidly expelled at the sides of the neck, the action, as seen in 

 an ordinary fish, being accompanied by regular movements of deglutition, and 

 by a characteristic opening and shutting of the opercula. In the Cartilagin- 

 ous Fishes, the water passes in streams from the lateral openings, which are 

 sometimes more or less valved. Drawing a fish rapidly backwards in the 

 water, may asphyxiate it, by bending up the branchial fringes, if the opercula 

 be open, or by exclusion of water, if these be closed. Each fringe-like process 

 of a gill-plate, is supplied by a branch of the branchial artery, which brings 

 dark or venous blood from the heart and bulbus arteriosus ; this divides into 

 minute vessels, which end in the branchial capillaries; from these again, the 

 branchial veins arise, which pass back to the base of the gill, and all combine 

 to form the aorta. In passing through the gills, the blood is oxygenated. As 

 the heart of the Fish is branchial, all the blood returned from the body is 



